The extraction yields of Np(E_Np) by solvent extraction with tribuyl phospate in n-dodecane(TBP/NDD) increased with concentration of HNO₃ regardless of oxidant. Without oxidant, E_Np was about 12% at 2 M HNO₃ and increased to 56% at 4 M HNO₃ due to the disproportionation of Np(V) to Np(VI) and Np(VI). Adding the NH₄VO₃ of 1 g/L, as a oxidant, E_Np was about 75% at 2M HNO₃. This was attributable to the oxidation of Np(V) to Np(VI). In the presence of U or Zr, E_Np decreased with concentration of U and was about 66.3% at U of 10 g/L. On the other hand, E_Np had no effect with concentration of Zr and was in the range of 88±2%. It was found that E_Np adding Zr was more enhanced about 13-15% than that of Np only. In the presence of both U and Zr, E_Np also had no effect with concentration of Zr and slightly decreased with concentration of U. When U was less than 5 g/L, E_Np was in the range of 88±2% by the synergetic effect of Zr only. At U of 10g/L, however, E_Np was 82±1% by the synergetic effect of Zr and decreasing effect of U. In the 9 components system containing the NH₄VO₃ of 1g/L, the extraction yields of Np and U were about 89% and 93%, respectively, and those of Fe, Mo, Zr, Y, Cs and Sr were below 5%. Therefore, Np and U could be effectively co-separated from the simulated HLW solution by controlling the oxidation state of Np.